D FACTORS AFFECTING VEHICLE SKIDS: A BASIS

FOR WET WEATHER SPEED ZONING

SUMMARY REPORT of

Research Report 135-2 Study 1-8-70-135

WET X/)

WEATHER SPEED LIMIT

50 Cooperati.ve Research Program of the

Texas Transportation, In,sti.tute and the Texas Highway Department In Cooperation with the

U. S. Department of Transportation, Federal Highway Administration

February, 1973

TEXAS TRANSPQJ:iTATION INSTITUTE • 8> 6 ..,.. q.,

Texas A&M University College Station, Texas

9

Factors Affecting Vehicle Skids: a Basis for Wet Weather Speed Zoning

by

G. D. Weaver, K. D. Hankins, and D. L. Ivey

Tl"' 1:1111dmc11/ of T<:rns Sr 1111/e Uill No. !S1, Ser/ion 167 !,as /J/({(cd 11j,011. lf"' S/11/e f-liglrn'll)' Co111111i.1.1im1 il,e 1111//l(ni/y (llld u:.1/)()nsibilily

lo t:.1la/,Lis/1 /'l'/11(11111/,/c /Ill(/ s11f1· sj//'{'{/ li/1/ils when co11dilio11s mused /J\'

wet or i11c!cn1c1tl 11 1r:allu:r n'quiu: ,\11c/1 adion. T/1is u:/J<rrl s'!nn111ari~:.cs

Nesc1irc/1 1/c}!orl !l'i-'.l, Study !-S-70-/15, 11•!1ir/1 /n1·.11:11/.1 a lll('//1od, /,a.1('{(

011 l/11• fl.\.1i111iflllio11 of //'1'11i/11/;/e i11forn111/io11 frn111, ,111-iious skid-u,fllteil

tt'sr:rnch, by 1u/1ich ·11·cl t.1't'a/J1cr .\f,ced --:,nni11g 111ny lJr: imfJftnnc11/er( in "f'cx11s.

Introduction

Reducinq tho hurnan and economic losses duc: to wet-weather skiddinq accidents is a of lhe Texas Highway Department cmd the Fc,dc:ral Administration. Allhouc1h the hic1hway surfm:p is only one o/ LhP problem, pavement slipperiness seems to be the factor receivinq attention in some sectors. In order to available infor-mation the various factors must bo kept in proper This is 1-8-70-135, tho coordinatinq in the by re 1. As

task within 1 Weather Zon-report has made use of inforrnotion from the individual

in this proqram to form a basis for im-plementing- wet weather speed zoning at selected sites in Texos in response to Senate Bill 183, Section 167.

factor in many wet weather accidents. realizes that he must reduce his

when the is wot if he is to maintain vehicle control conditions. Unfortunately, tho

for safe rnay not be apparent.

Since the potential for skiddinq is so speed-sensitive, estab­lishment of wet weather limits represents one approach toward attackinq the wet weather skiddin9 problem. Other corrective measures such as geometric and surface improve-­ments and intensive driver education are obviously warranted in many cases. However, these measures represent long-term objectives in the total skid reduction program, whereas wet weather speed zoning offers the possibility of relieving the im­mediate problem in priority locations.

The establishment of wot weather speed limits is based on . an engineering and traffic investiqation." The primary

objective of this study was to assimilate pertinent findings from

Figure I. Vehicle-roadway interaction pro,gram in Texas.

various skid-related research efforts to provide an objective basis on which potential wet weather accident sites can be analyzed and hence, safe wet weather speed limits may be determined.

Friction Availability Versus Friction Demand

The performance of desired maneuvers is dependent upon the existence of tire/road friction. It is well known that the fric­tion required (demand) by a vehicle to perform a given maneu­ver increases with speed. On the other hand, the friction avail­able to the vehicle (skid resistance) at the tire-pavement inter­face normally decreases with increased speed. Loss of control usually occurs when the friction demand exceeds the friction available. The friction at the point where availability and de­mand are equal is defined as "critical friction," and the speed at which this occurs is termed the "critical speed," The critical friction concept is used throughout this report as a basis for evaluating the individual factors that influence friction demand.

Available Friction

The skid number (SN) determined by the locked wheel skid trailer at 40 mph is a widely accepted measure of pavement friction. Friction measurement by this method, however, is

100

80

z en

i 0 60 ;: 0 ii: .._ w ..l al

" 40 ..l

~ "

20

0

0 10 · 20 30

rEXTURE: 0.050 IN. OR LESS TIRE TRE~D, 2/32 IN. OR LESS

AVAILABLE FRICTION FOR 85th PERCENTIL~ RAINFALL CONDITIONS

40 50 60

SPEED, MPH

70 80

Figure 2. Available friction as predicted by skid number (2).

usually obtained at the one speed, 40 mph, and as mentioned previously, friction decreases as vehicle speed increases. Since the speeds in question here are usually in excess of 40 mph, the change in available friction with respect to speed must be considered. Figure 2 presents friction/speed relationships and is based on the 85th percentile rainfall intensity of 0.14 inches/ hour, pavement texture of 0.05 inches and tire tread depth of 2/32 inch. The research report contains general nomographs by which similar curves can be developed for various parameter combinations.

It should be noted that in Figure 2, the intersection of a ver­tical line at 40 mph with the SN40 = 40 mph curve does not yield a skid number of 40 when projected horizontally to the available friction ordinate. Although this, at first, may appear question­able, the reason lies in the fact that the curves were developed from skid data obtained under external watering conditions (rain machine) rather than the standard ASTM internal trailer water­ing system. The curves in Figure 2 more closely reflect pave­ment characteristics under actual wet weather conditions.

Demand Friction

Having determined the relationship of available friction with speed, the other part of the problem involves equating the de­mand friction for traffic maneuvers to this from which the critical speed may be determined. Demand friction relationships are provided for the maneuvers listed below. The development of

100 FN = V2 5 0.3 (d-3.67V)

~

ci 80 w <D :, ::, z

z 0 ;:: 60 u ii: IL

z U)

40 -0: w <D :, ::, z

0 20

"' "'

faample 2

0 0 10 20 30 40 50 60 70

SPEED, MPH

Figure 3. Critical speed for emergency stop imposed by sight distance and available friction (3).

these relationships is discussed in considerable detail in the research report.

l. Stopping maneuvers (Figure 3).

2. Cornering maneuvers (Figures 4 and 5).

3. Passing maneuvers (Figure 6).

4. Hydroplaning (Figure 7).

5. Emergency path-correction maneuvers (Figures 8, 9, and 10).

The critical speed is determined by the intersection of the demand friction curve and the appropriate skid number curve.

Rather than developinci a series of curves for selected de­grees of superelevation, the curves in Figures 4 and 5 are devel­oped on a zero-superelevation basis to which appropriate cor­rection factors may be easily applied for a desired supereleva­tion rate. To include the effect of superelevation, the given demand curve, FNc, is translated vertically by the amount of the superolevation expressed in percent. As an examplo, if a 15-degree curve contained 0.0:i positive superelevation, the 15-degree curve would be lowered 5 units of FN as shown by the dashed curve in Figure 4. Similarly, the curve would be trans­lated upward an equal amount if the superelevation were negative.

100 20•

FNc _ v2

z e + Tcio -ISR 15°

lL_

.ffi 80 <D ::;; :::, z

w z a:

~ 60 :::,

t !.2 10° ii: a: lL :::,

u

lL

7• 0 40

s• w z w (J) a:

5• <.!) er w w 4• Cl <D

~20 3• z Cl 2• ,2 (J) 1•

0.5° 0

0 10 20 30 40 50 60 70

SPEED, MPH

Figure 4. Critical speed on horizontal curves (smooth transition, zero superelevation) ( 4).

Figure 4 should be used for spiral transitions or at locations within a curve. Figure 5 should be used for determination of critical speeds on curves having abrupt transition regions at either end (situations normally found when spiral or compound curve transitions are not provided).

Wet Weather Speed Zoning Design Process

The design process involves equating the available friction at the selected site to the friction demand for traffic operational maneuvers expected at that site. To do this, certain engineering characteristics of the site must be known from which the avail­able friction may be determined. Similarly, certain traffic oper­ating characteristics must be determined. Engineering and traf­fic ch.aracteristics necessary for site evaluation are discussed in the research report.

A critical speed is determined for each expected maneuver. The wet weather speed limit will be governed by the expected maneuver producing tho lowest critical speed. Examples are presented in this report to illustrate the design procedure for selecting the wet weather speed limit.

The design process may be accomplished by the following procedure:

1. Determine engineering characteristics of the site. 2. Identify the expected traffic operating characteristics and

expected maneuvers.

100

z tL

ci ~ 80 :;; ::, z z 0

t 60 ii tL

40

2 <J)

ci w CD ::;, 20 :, z 9 "' <J)

0

0 10 20 30 40

SPEED, MPH

50 60

10•

.,. e• 5•

4• tL

3• 0

2• w w 0:: (!) w 0

0.5°

70

Figure 5. Critical speed on horizontal curves ( abrupt transition, zero superelevation) ( 4).

3. Determine the available friction at the site including the eHods 0£ spood by: (a) conducting standard skid trailer meas­urements at 40 mph and tho of SN and speed from Figure 2, and (b) conducting tests with a skid trailer at 20, 40 and GD rnph using an external watering system such as a water truck and developing friction relationships.

4. Delermine the critical speed for the maneuvers from the applicable speed-friction relationships shown in Figures 4 through 10

5. I! combination maneuvers are expected at a site, deter­mine the critical speed for the combined maneuver by the pro-cedure discussed in 1 of this report.

6. Select the lowest critical dclerrnined from Steps 4 or 5. The wet weather speed limit is established as this value. The posted speed would be tlw wet weather speed rounded to the nearest 5-mph increment.

Site Selection Criteria

The followinq criteria aro sugcicstcd for site selection:

l. Any Control-Section exhibitincJ two or less wet weather accidents annually should not be considered.

2. All Conlrol Sections exhibitinq 20 or more wet weather accidents annually should be considered.

3. For Control-Sections having 3 to 19 wet weather acci­dents annually, the Control-Section or sites within the Control­Section should be considered if:

Daily Vehicle Miles a. No. of Wet Weather Accidents ~ 3,000

b. The Control-Section length is 0.30-mile or more.

Examples of Wet Weather Speed Determination Several examples are presented to illustrate the design

process for establishing the wet weather speed limits at sites exhibiting different engineering and expected traffic operating characteristics.

Example I: Critical Speed for Combined Man.euvers Many common maneuvers include some combination of

acceleration, braking, and cornering. The total friction demand may be determined by vector summation of the friction demand for the individual maneuvers. The following example illustrates the manner in which the critical speed may be determined for a combination maneuver. This example illustrates the procedure to determine the critical speed for a maneuver involving com­bined braking and cornering such as might be experienced at an exit ramp to a stop-controlled service road.

Given: Engineering studies revealed the following site char­acteristics:

I. Available friction, FN = 30.

2. Available stopping sight distance, SD 400 ft.

100

ii' cc ., m

" ~ 80

z 0

t; °' "- 60

40

~ cc ., ~ 20

~ 0 E111mple3

~ E~ample 2

0

0 10 20 30 40 50 60 70 80

SPEED, MPH

Figure 6. Critical speed for passin.g maneuvers (5, 6).

3. Ramp contains spiral transition curve to a 20-degree maximum curvature with no superelevation.

Procedure: 1. Make an initial assumption of critical speed. V0 = 30

mph.

2. From Figure 4 (Use Figure 5 for an abrupt transition.) and the friction demand curves, determine the friction number demand for cornering, FNc, using the assumed speed, V 0. As shown by the dashed line in Figure 4, FNc = 22.

3. From Figure 3, determine the friction number demand for stopping, FN., using the assumed speed, V0 • As shown by the dashed line in Figure 3, FN2 = 10.

4. Compute the total friction number demand for the com­bined maneuver, FNt. The total friction demand is the vector sum of the cornering demand, FNc, and the stopping demand, FN •.

FNt y FNc2 + FN.2

v (22)2 + 00)2

24.2

5. Since FNc, FN., and hence, FNt, are dependent on the assumed speed, the resultant interaction point (point having coordinates V0 , FNt) must be located in Figure 2. If the point lies above the available friction curve applicable to the site (in this case, the SN40 = 30 curve), a lower initial speed, V0 , must be assumed, and the above process (steps 1 through 4) repeated. Similarly, if the point lies below the applicable available friction

:c 0.. ~

60

50

fi 40 ... 0..

"'

30

0

TEXTURE =0.15

TEXTURE =0.10

________________ T!_!E:_!X~TU~R!!:E_:=~0_:!.05!!_ ___ _

0.1 0.2 0.3

TEXTURE ,,; 0. 02

TIRE TREAD: 2/ 32 IN. OR LESS TIRE PRESSURE: 27 PSI

0.4 0.5 0.6

WATER DEPTH, INCHES

0.7

Figure 7. Critical hydroplaning speed imposed by water depth and pavement texture (7).

curve, a higher speed, V0 , must be ossumed, and the process repeated. The critical speed (the speed at which the point falls on the applicable SN-versus-speed curve) may be closely ap­proximated in twq or three ~rials.

6. Plotting the interaction point having coordinates V0 = 30 and FNt = 24.2 on Figure 2 reveals that the point lies sl~ghtly below the applicable SN40 = 30 curve (Point A, Figure 2). Therefore, a higher ::;peed, V0 = 35 mp'.1 ·was assumed, and the process repeated. For V0 = 35 mph, FN0 = 30 (Figure 4), FNs = 15 (Figure 3), and FNt = 33.5. The interaction point (coordi­nates 35, 33.5) is plotted as Point Bin Figure 2 which lies slightly above the SN40 = 30 curve. A straight line between Points A and B indicates a critical speed for the combination maneuver of approximately 32 mph (speed at Point C).

Example 2: Critical Speed on Tangent Section

The following site characteristics are assumed for illustra-tive purposes:

1. SN40 = 40. 2. Sight distance, SD = 700 ft.

3. Level tangent highway section having no paved shoulders.

4. Pavement texture = 0.05. 5. Pavement exhibits good drainage with no evidence of

rutting or ponding.

Procedure: l. Identify the traffic maneuvers that would be expected

at the site. The expected maneuvers would include: (a) stop­ping maneuvers, (b) passing maneuvers, and (c) emergency correction maneuvers.

2. Since the site is a level tangent section, cornering or combination maneuvers would not be expected. Therefore, critical speeds for these maneuvers would not be applicable at this site. Similarly, since there is no evidence of rutting or ponding, and good drainage is provided, critical speed to pro­duce .hydroplaning is not applicable at this site.

3. From Figure 3, the critical speed for a stopping maneu­ver (SN40 = 40, SD = 700 ft) is 59 mph.

4. From Figure 6, the critical speed for a passing maneuver (SN40 = 40, SD = 700 ft) is 59 mph.

4. From Figure 6, the critical speed for a passing maneuver (SN40 = 40) is 51 mph.

5. From Figure 8, the critical speed for an emergency path correction (SN40 = 40, no paved shoulders) is 52 mph.

6. The lowest critical speed from steps 3 through 5 -is 51 mph, governed by friction demand for a passing maneuver.

7. Rounding off to the nearest 5-mph increment, the wet weather speed limit would be 50 mph.

Example 3: Critical Speed on Horizontal Curve

The following site characteristics are assumed for illustrative purposes:

1. SN40 - 35. 2. Horizontal curvature - 3 degrees with an abrupt transi­

tion from tangent section to the circular curve (that is, no spiral was used at the transition).

3. Superelevation, e, = 0.05 percent.

4. Seal course pavement surface is slightly flushed in the wheel paths. The texture in the wheel paths is 0.020 as com­pared to 0.065 at other locations. The flushed wheel path is considerably wider throughout the curve than on the tangent approach.

5. The pavement grade is 0.4 percent.

70

NO PAVED SHOULDERS

60

50

FRICTION DEMAND FOR EMERGENCY

PATH CORRECTION

z 40 Cl)

z 2 1--~ 30 a:: u..

20

10

20 30 40 50 60 70 SPEED,MPH

Figure 8. Critical speed for emergency path corrections (3) (two-lane highways, no shoulders).

6. The pavement surface is slightly rutted in transition area from normal crown to superelevo1ed section. Based on string­line measurements of rut depth, obsei·valion of a flat area in the superelevation transition, and the differential texture between the wheel path and surrounding SlJrface, expected water depth 1s approximately 0.160 inches.

7. Sight distance, SD, is in excess of 1000 ft.

Procedure:

1. Identify the traffic manc-,uvers that would be exi::;ected at the site. These would include: (a) passimJ maneuvers and (b) cornering maneuvers.

7.. Since appreciable water depths are expected and rutting is evident, the critical speed for hydroplaning should be determined.

3. Since adequate sight distance is available, stopping maneuvers are not critical.

70

60

50

Z 40 (/)

z 0 f--2 a:: 30 LI..

20

10

0

20 30

I - 6' PAVED SHOULDERS

40 50

SPEED,MPH

FRICTION DEMAND FOR EMERGENCY PATH CORRECTIONS

60 "TO

Figure 9. Critical speed for emergency path corrections (3) (two-lane highways, 1-6 ft. shoulders).

70

60

50

~40

20

10

0

20 30

6·10' PAVED SHOULDERS

40 50

SPEED , MPH 60 70

Figure 10. Critical speed for emergency path corrections (3) (two-lane highways, 6-10 ft. shoulders).

4. From Figure 5, the critical speed for cornering (SN40 = 35, D = 3°, e = 0.05), is 68 mph.

5. From Figure 7, the critical speed for hydroplaning (tex­ture = 0.02, water depth = 0.160 in.) is 40 mph.

6. From Figure 6, the critical speed for a passing maneuver (SN40 = 35) is 45 mph.

7. The lowest critical speed determined in steps 4 through 6 above is 40 mph, governed by hydroplaning.

8. The wet weather speed limit would be 40 mph.

References

1. Weaver, G. D., Hankins, K. D., and Ivey, D. L. "Factors Af­fecting Vehicle Skids: A Basis for Wet Weather Speed Zon­ing." Res. Rept. 135-2F, Texas Transportation Institute, February 1973.

2. Hankins, K. D. Unpublished Texas Highway Department in­house working documents.

3. Glennon, J. C. "A Determination Framework for Wet Weath­er Speed Limits." Res. Rept. 134-8F, Texas Transportation Institute, August 1971.

4. Glennon, J. C. and Weaver, G. D. "The Relationship of Ve­hicle Paths to Highway Curve Design." Res. Rept. 134-5, Texas Transportation Ins ti tu te, May 1971.

5. Glennon, J. C. "Friction Requirements for High-Speed Pass­ing Maneuvers." Res Rept. 134-7, Texas Transportation In­stitute, July 1971.

6. Weaver, G. D. and Glennon, J. C. "Passing Performance Measurements Related to Sight Distance Design." Res. Rept. 134-6, Texas Transportation Institute, June 1971.

7. Martinez, J. E., Lewis, J. M., and Stocker, A. J. "A Study of Variables Associated with Wheel Spin-Down and Hydroplan­ing." Res. Rept. 147-1, Texas Transportation Institute, March

1972.

The published version of this report may be obtained by addressing your request as follows:

R. L. Lewis, Chairman Research and Development Committee Texas Highway Department-File D-8 11th and Brazos Austin, Texas 78701 (Phone 512/475-2971)